1. Field of the Invention
The present invention relates to a photometering circuit applicable to optical equipment such as a camera.
2. Related Background Art
In optical equipment such as a camera, a photometering circuit which uses a photo-diode as a photo-sensing device has been used to measure a light intensity. The photometering circuit usually has a circuit configuration as shown in FIG. 1 which supplies a current flowing through a photo-diode PD in proportion to a light intensity, to a diode Q1 (which is a collector-base connected transistor in an IC) to produce a voltage across the diode Q1 so that the light intensity is logarithmically compressed to secure a wide dynamic range. In FIG. 1, V.sub.k denotes a reference voltage supply.
In the photometering circuit of FIG. 1, the photo-diode PD is connected to an operational amplifier OP with a cathode thereof connected to a reference potential and an anode thereof connected to an inverting input terminal The photo-diode PD may be connected in the reverse direction. In this case, the negative feedback diode Q1 is also connected in the reverse direction, but the basic concept of the circuit operation does not change. The following description is applied to the connection shown in FIG. 1.
The photometering circuit of FIG. 1 may cause a so-called latch phenomenon in which a potential at the inverting input terminal of the operational amplifier OP is lower than a potential at a non-inverting input terminal when virtual short-circuit condition across the input terminals of the operational amplifier OP is effectively lost due to an external noise, whereby a potential at an output terminal V.sub.out swings to a maximum level. In this case, since the diode Q1 which is the negative feedback element is fixed in an off state, it is not possible for the circuit to spontaneously return to a normal operation condition.
This phenomenon is unavoidable so long as the non-linear element is used for the feedback, and many proposals to quickly release the phenomenon have been made.
FIG. 2 shows a photometering circuit which includes a simplest device to release the latch. In the present circuit, a diode Q3 is added anti-parallelly to the negative feedback diode Q1. When the latch phenomenon is to occur, the output terminal V.sub.out functions to pull up the potential at the inverting input terminal through the diode Q3 so that the balance between the input terminals is immediately recovered to the normal operation condition.
However, the photometering circuit shown in FIG. 2 has a problem in a response speed of the output to a change of an input light and the response speed is lower than that of the basic circuit shown in FIG. 1 which has no latch release device. The reason is as follows
Since each semiconductor device comprises a P-N junction, a parallel junction capacitance is present across terminals When such a junction capacitance is taken into consideration, the photo-metering circuit of FIG. 2 which includes the latch release device is represented by a circuit shown in FIG. 3. A capacitor C1 is parallelly connected to the negative feedback diode Q1, a capacitor C2 is parallelly connected to the photo-diode PD, and a capacitor C5 is parallelly connected to the latch release diode Q3. The respective capacitances are approximately C1=C5=0.5 pF and C.sub.2 =100 pF. In order for the output voltage V.sub.out to change to follow the change in the input light intensity, the photo-current from the photo-diode PD must charge up the capacitors C1 and C5 parallelly connected to the diodes to a proper output voltage level, and the response of the output voltage is delayed by the time required for such charge. Since the delay time is affected by the photo-current and the capacitance of the feedback stage, the smaller the light intensity to be measured is and the larger the capacitance of the feedback stage is, the longer is the delay time. Since the capacitance of the feedback stage of the photometering circuit of FIG. 2 which includes the prior art latch release device is larger than that of the basic circuit of FIG. 1, the response speed is lower.
Although an absolute value of the capacitance of the capacitor C2 parallel to the photo-diode is large, the terminals are virtually short-circuited during the normal operation of the operational amplifier OP and the bias voltage does not substantially change even if the light intensity changes. Specifically, it changes only for 1/open-loop gain of the output voltage change. Accordingly, the effect on the response speed is usually smaller than that by the capacitance of the feedback stage.
The prior art apparatus thus includes a problem of lowering the response speed of the output to the change of the input light when the latch release device is added to the photometering circuit.